1.Field of the Invention
The present invention relates to an interference microscope.
2.Related Background Art
A reflected light differential interference microscope will be explained by way of one example of a prior art interference microscope with reference to FIG. 6. In the differential interference microscope in FIG. 6, a beam of illumination light emitted from a light source 1 is converged by a collector lens 2 and thereafter becomes a beam of linearly polarized light in a polarizer 40. Then, the beam of linearly polarized light is reflected by a beam splitter 41 and thereafter travels through a Wollaston prism 42. The beam of linearly polarized light is, when penetrating the Wollaston prism 42, separated into ordinary ray 45 and extraordinary ray 46 by action of birefringence. Both of those ordinary ray 45 and extraordinary ray 46, of which vibration directions are perpendicular to an optical axis 44 and orthogonal to each other, are linearly polarized light. The thus separated ordinary ray 45 and extraordinary ray 46 are each given a slight separation angle when penetrating the Wollaston prism 42, and are therefore collimated with a slight spacing from each other by action of convergence of an objective lens 3. The collimated rays of light respectively fall upon slightly separated positions on a subject 4.
The two rays 45 and 46 respectively reflected on from the subject 4, converge on the Wollaston prism 42 by the action of convergence of the objective lens 3, and again travel on the same optical path by the action of birefringence of the Wollaston prism 42. Then, the rays 45 and 46 pass through the beam splitter 41, and an analyzer 43 fetches only equi-directional vibration components of the rays of linearly polarized light orthogonal to each other in the two rays 45 and 46, and these two rays interfere with each other, thereby forming an image 5. In the image 5, interference fringes corresponding to a phase difference given to the two rays 45 and 46 when reflected in positions slightly different from each other on the subject 4, are observed.
The image 5 is a so-called differential image, and a phase slope can be observed as a relief image (a protruded/recessed image) or an interference color image due to brightness and darkness of the image depending on an observation condition. Further, it is a classical method to quantify a phase distribution from an interference color of the interference color image. According to this method, the interference color observed is compared with a previously prepared interference color chart showing a relation between the interference color and the phase difference, thereby obtaining a phase difference .DELTA. between the two rays 45 and 46 at a point of the observation. Herein, a slope angle .theta. of the subject 4 at the point of the observation is given by: EQU tan .theta.=.DELTA./(2S)
where S is a shear amount (a separation breadth between the two rays 45 and 46 on the subject 4) of the differential interference microscope. PA1 where d is a width of the slope portion containing the point of the observation of the subject 4.
Further, a phase difference t with the whole slope can be calculated such as: EQU t=d.multidot.tan .theta.
Moreover, if a phase difference substantially equal to a central wavelength .lambda. of the white light, is given between the two rays 45 and 46 reflected by the subject 4, the image 5 assumes a red purple color. This is called a sensitive color image. Such a red purple color image can be formed by sliding the Wollaston prism 42 in a direction of wedge angle, or by inserting one wavelength plate into the optical system. In the sensitive color image, the interference color changes sensitively to a slight change in phase, and hence a phase difference of the subject 4 can be estimated with a high accuracy.
In the conventional differential interference microscope described above, however, the protrusion and recess of the relief image reversely appear and the interference color appears differently, depending on a polarizing direction of the two rays 45 and 46 or on how an orientation of an analyzer 43, is set. It is therefore difficult to judge whether the phase slope is positive or negative. Further, there arises a problem in which the estimation of the phase difference on the basis of the interference color in the prior art has a large scatter in terms of a result due to a subjectivity of the observer, an error in print colors of an interference color chart, a spectral characteristic intrinsic to the optical system and so on. Another problem is that the estimation is time-consuming.
For obviating these problems, for example, Japanese Patent Application Laid-Open Nos.5-232384, 9-105864 and 10-104524 disclose methods of taking in a plurality of images with changes in the observation conditions while changing the phase difference of the image by moving a component constituting the apparatus, and calculating the phase slope and the phase difference with a high accuracy by executing an arithmetic process with use of the data about the plurality of images. According to those methods, however, it is required that the optical components of the apparatus be electrically movable, and besides the complicated arithmetic process be executed.